1. Field of the Invention
The present invention relates to a high bandwidth, high efficiency power supply system that can be used to power an RF PA (Radio Frequency Power Amplifier).
2. Description of the Related Arts
In certain electronic systems, there is a need for a high bandwidth, high efficiency power supply. As an illustrative example, a radio system may include an ET (envelope tracking) transmitter, where the RF PA is fed by a power supply which tracks the amplitude of the RF modulation. The RF PA is therefore supplied with a varying voltage that improves efficiency. Typically, the power supply providing power to such a PA is a linear regulator with a fast response, with the output voltage of the linear regulator controlled electronically to track the amplitude modulation. However, such a linear regulator is inefficient, as linear regulators control the output voltage via a dissipative pass transistor. A more efficient alternative could be to use a switching regulator. However, these switching regulators lack the control bandwidth to modulate their output voltage at the rate needed to track the amplitude modulation in many modern radio systems.
A class G amplifier may be used to provide power to a PA. A conventional class G amplifier architecture improves the efficiency of a Class AB amplifier, by allowing the class G amplifier to switch between multiple power supply rails during operation. Typically, the lowest voltage power rail is chosen while the signal swing is low, and higher voltage rails are used when the signal swing is higher. Thus, the conventional class G amplifier is supplied by the higher voltage rails only as needed. The power efficiency of a class G amplifier is a strong function of the voltage of the supply rail, since the power equal to the difference between the voltage of the supply rails and the voltage at the amplifier output multiplied by the output current is dissipated as heat. The best efficiency is thus achieved by switching to the lowest voltage supply rail possible while maintaining sufficient headroom for proper operation of the amplifier.
FIG. 1 shows a conventional unipolar Class G amplifier. The amplifier 100 is unipolar because only positive supply voltages rails are used. Voltage supply rails V1 and V2 have associated drivers 102 and 104 capable of sourcing current to the amplifier output 112, when enabled. Voltage V1 is assumed to be lower than voltage V2. Driver 132 sinks current from the load (not shown) at the output 112 to ground. At any point in time, either driver 102 or driver 104 is selected to operate, depending on the voltage at output 112. Comparator 118 compares this output voltage 112 with a threshold voltage 122 at its negative comparator input, to select either driver 102 or driver 104 via select signals 106, 108, respectively. Inverter 125 ensures that, when one driver (102 or 104) is on, the other driver (104 or 102) is off. Threshold voltage 122 is approximately equal to voltage V1 but may be offset slightly by offset voltage 120, so that driver 104 is selected (and driver 102 is unselected) when output voltage 122 is slightly below voltage V1, to provide for some voltage headroom for driver 102. As described earlier, compared with other conventional amplifiers operating from a fixed voltage V2, the conventional Class G amplifier of FIG. 1 improves efficiency to some extent by switching to a lower voltage supply rail V1 whenever the output voltage 112 is low, thereby reducing dissipated heat.
Class G amplifiers may be used in a variety of configurations, including bipolar schemes (with positive and negative supply rail voltages), as well as in bridge amplifiers. However, Class G amplifiers are typically limited to low frequency operation (e.g. audio). This well-known limitation is due to distortion increasing in the amplifier with increasing frequency. When the Class G amplifier switches supply rails, a discontinuity in current results from the selection and deselection (enabling or disabling) of the drivers 102, 104. At low frequencies, this discontinuity is relatively short compared with the signaling periods, and thus the energy from the glitch makes a relatively low contribution. However, at higher frequencies, the glitch period becomes proportionally more substantial. For example, today's conventional Class G amplifiers used for an ET (envelope tracking) transmitter, where the RF PA is fed by a power supply which tracks the amplitude of the RF modulation, may not be suitable for many transmitter systems operating at modulation rates common in cellular systems. In cellular systems, distortion causes unwanted out-of-band spectral energy that reduces network utilization, and may exceed the limits specified in the specifications required for such systems. For example, the 3G WCDMA system utilizes a modulation symbol rate of 3.84 MSPS (megasymbols per second), at least two orders of magnitude higher frequency than audio.